Article pubs.acs.org/JPCC
Solvent-Adaptable Poly(vinylpyrrolidone) Binding Induced Anisotropic Shape Control of Gold Nanostructures Abhitosh Kedia and Pandian Senthil Kumar* Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India S Supporting Information *
ABSTRACT: Conformational changes in the intrinsic chemical structure of the polymer poly(N-vinyl-2-pyrrolidone) (PVP) in aqueous as well as (in)organic solvents essentially dictates the novel room temperature seedless synthetic procedure for the reduction of hydro-chloroauric acid (HAuCl4.3H2O) leading to the formation of different anisotropic size/ shaped gold nanoparticles. The interaction between gold metal ions and PVP at the given specific monomer to metal ratio leads to sequential metal ligand exchange, thereby simultaneously utilizing the mild reducing property as well as distinct structure-directing/capping ability of PVP in different (in)organic solvents, the synchronized features of which have been carefully explored through NMR and FTIR measurements identifying the foolproof signatures of the polymer coordination interaction for the first time in designing the systematic nucleation and growth/stabilization procedures of anisotropic metal gold nanostructures. Furthermore, the complementary XPS data evaluates the quantitative role of coupled oxygen and nitrogen components of the pyrrolidone ring in the PVP−solvent complex in asserting seedless surface mediation as well as the morphology driven localized surface plasmon suitable for wide range of plasmonic as well as photonic applications.
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INTRODUCTION The upsurge in research activities on the synthesis of nanogold and nanosilver with controlled geometries (such as sphere, cube, rods, polyhedral, belts, nanorice, nanokites, platelets, nanotedpoles, prisms, decahedra, and nanostars1−8) is motivated by the size/shape tunable optical properties of these metallic nanoparticles, which have many projected applications in nanoelectronics, catalyst, optical filters, photon transport, and surface-enhanced Raman or fluorescence scattering.8−16 Any physical/chemical method utilized for producing low dimensional metal nanostructures require precise tuning of nucleation and growth steps to achieve crystallographic control.5,17−21 Shape controlled metal nanocrystals synthesized by wet chemical methods possess well-defined surfaces and morphologies because of their controlled nucleation at atomic levels followed by their stabilized growth. The morphology of the resultant thermodynamically unfavorable size/shape controlled metal nanoparticles has already been explained in terms of particular environments in which they are synthesized.17−19,22−24 The use of capping agents generally results in the anisotropic growth, allowing shapes other than spheres to be controllably synthesized. The capping agents preferentially adsorb on certain crystal facets, so that the growth lacks/proceeds only along certain specific crystal faces, thereby defining the final nanoparticle size/shape.5,17−19,25,26 Additional control over the nanoparticle shape can be obtained by separating the nucleation © 2012 American Chemical Society
and growth processes, where small seed particles are nucleated first and these seeds are grown into larger nanoparticles through a slower reduction process in the presence of capping agent. PVP is a typical homogeneous amphiphilic polymer containing a strong hydrophilic component (the amide group) and a significant hydrophobic moiety (six carbons per monomer unit) suitable not only for stable surface stabilization but also renders mild reducing power toward the formation of various metal nanoparticles.17,21,27−29 An important feature of PVP is the presence of strong electronegative oxygen adjacent to the nitrogen atom in carbonyl group, which forms hydrogen bonding with surrounding solvent molecules.30−34 PVP is remarkably a stable polymer with its inert physicochemical properties over a wide range of pH/salt content but strongly depends upon the surrounding fluid environment (solvent). The interaction of PVP with metal ions has been well studied,1,35,36 but none of these works really assimilated the solvent interaction with PVP which has been reiterated as a crucial factor (very recently by our group37 and some others also38−40) in differentiating not only the kinetic/thermodynamics of metal nanoparticle synthesis but also vital in delineating the exclusive dependence of the external energy sources in manipulating the morphology of the nanocrystals. Received: July 13, 2012 Revised: October 10, 2012 Published: October 15, 2012 23721
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corresponding PVP-solvent medium as well as with pure PVP powder, wherever necessary.
On the basis of this premise, we propose that, even under normal conditions, this dynamic solvent−solute interaction along with the their different coordinate geometries dominates the nucleation regime in precise and further plays a significant role in polymer chemisorption onto the surface of the metal nanoparticles directing them to anisotropically grow with different intriguing sizes and shapes, which has been lacking in the literature reports.18,29,35 Herein, we report our coherent experimental results based on the combined NMR, FTIR, and XPS measurements on the real time interaction between PVP and the dispersed solvent in the presence of metal ions, revealing not only the kinetic/ thermodynamic strength of the individual components in the pyrrolidone ring of PVP, which characteristically gets adsorbed onto specific metal ion/nanocrystal surfaces but also signifies a fine paradigm for the microscopic characterization of the nucleation/growth/stabilization of complex size/shaped metal gold nanostructures, which has not been properly understood in its true sense till now.
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RESULTS AND DISCUSSION In all our present synthesis procedures, the molar ratio of PVP to metal gold ions was kept constant at ∼3250, the only variant in the reaction protocol being the individual solvents used, ensuing the respective changes in the optical properties of the as-formed gold nanocrystals (strongly dependent upon particle size/shape) as clearly illustrated in Figure 1.
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MATERIALS AND METHODS In a typical synthesis, 0.27 mM aqueous solution of hydrochloroauric acid (HAuCl4.3H2O, Aldrich) was mixed with 15 mL of 10 mM polyvinylpyrrolidone (PVP average MW = 10 000, Aldrich) solution in methanol, ethanol (EtOH), chloroform (CHCl3), 1-propanol, 2-propanol, and dimethylformamide (DMF). The change in color of solutions (ranging from pink to blue to violet) indicates the formation of anisotropic gold nanostructures. All experiments and the various sample preparation procedures were carried out at room temperature, unless otherwise mentioned. Optical absorption measurements were carried out in all our as-prepared nanoparticle solution samples in the wavelength range of 200−1000 nm using Thermo Scientific absorption spectrophotometer. Aliquots of solution samples were dropped on a zinc selenide (ZnSe) plate, and their infrared measurements were carried out over the wavenumber range of 650− 4000 cm−1 using PerkinElmer RX1 Spectrophotometer. The 13 C NMR spectra were recorded on Bruker Avance-400 spectrometer using TMS as the internal standard and CdCl3 as a NMR solvent (chemical shifts in ppm). TEM samples were prepared by drying the 5-fold centrifuged samples in ethanol at around 4500 rpm (to remove excess PVP) on carbon Formvar coated copper grids and the images were acquired using the FETechnai G2 system operated at an accelerating voltage of 300 kV. Samples for XPS were prepared by drop casting the 5-fold centrifuged samples on silicon substrates, and measurements were carried out using Thermo Kα XPS instrument at a pressure better than 10−9 Torr. The core-level spectra of C 1s, O 1s, N 1s, and Au 4f were recorded using Mg Kα radiation (photon energy = 1253.6 eV) at a pass energy of 50 eV, an electron take off angle of 90°, and a resolution of 0.1 eV. The core-level spectra were background corrected using Shirley algorithm, and chemically distinct species were resolved using a nonlinear least-squares fitting procedure. The C 1s core-level spectra at 285 eV were taken as reference for the charge correction in other core-level spectra. All of the above-mentioned spectroscopic data were baseline corrected individually and manually stacked one above the other in a single plot, just for the sake of visual comparison only and should not be confused for overall normalization. Each data has been relevantly discussed in the text with respect to the
Figure 1. UV−visible absorption spectra of the as-formed gold nanoparticles in their respective solvents for the same monomer to metal ratio (∼3250) clearly differentiating the size/shape effect on the surface plasmon resonance.
In the alcohol medium, such as methanol and ethanol, utilized in the present case, though a single surface plasmon band centered around 530−540 nm is clearly visible (Figure 1), their characteristic color of the colloidal solution (red in methanol and pink in ethanol, see inset of Figure 1) distinguishes the representative polycrystalline quasispherical as well as the 5-fold asymmetrical decahedral gold nanostructures as seen from the corresponding TEM images (Figures 2 and 3). In chloroform, propanol, and DMF solvents, the presence of two surface plasmon bands (one at around 540 nm and other in the range 700−900 nm) indicates the general formation of anisotropic/elongated nanoparticle shapes, further identified as polyhedral (purple in chloroform), anisotropic multibranched (violet in propanol), and 3D hyper-branched tip like (blue in DMF) nanostructures as seen from their individual TEM images (Figures 1−3). The transverse plasmon band at lower wavelength (higher energy) arises due to the core of metal nanoparticles, whereas the longitudinal band at higher wavelength (lower energy) occurs due to the surface tips/edges (Figure 3). Moreover, the number, sharpness, and length of the tip like structures are far more in DMF solvent in comparison with other already known anisotropic branched/multipod gold nanostructures, which by itself extends the longitudinal plasmon band toward the near IR region (Figure 1), visually signifying their integral microengineered floral core design, an intriguing topology drastically different from the routinely faceted nanostructures, which are of current research interest worldwide due to their vast variety of potential applications, ranging from plasmonic nanoantennas to sensing.15,41,42 Essentially, the complex/irregular morpholo23722
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Figure 2. Low magnification TEM images (scale bar = 100 nm) of as-prepared anisotropic gold nanoparticles in different solvents (a) methanol, (b) chloroform, (c) ethanol, (d) 1-propanol, (e) 2-propanol, and (f) dimethylformamide.
conjunction with simultaneous active metal coordination, the efficacy of which has not been entirely looked upon till now, outlines the major key point of focus in our present synthesis protocol. Although the inherent mild reducing ability of PVP initiates the nucleation of gold seed particles, its active metal ion/particle coordination interaction not only instantly/strictly prohibits their random aggregation/agglomeration (quite common as in aqueous synthesis procedures)20,21,23 but also effectively dictates their shape evolution process (the kinetics of adsorption/desorption of PVP in various solvents is different for the different crystallographic planes of Au nanoparticles),17−19,25,43−46 summarizing the precise formation of various complex size/shape controlled metal nanostructures. For the basic understanding and elucidation of this intriguing nucleation/growth mechanism, one has to ostensibly include the polymer/solvent interaction,30−34 probed systematically for the first time through 13C NMR and FTIR measurements, the overwhelming results of which are incorporated in clarifying the final nanoparticle XPS measurements, as discussed in great detail in the following sections. From chemical thermodynamics viewpoint, generally for PVP in solution, the carbonyl group of the lactam as well as the monomer chain forms co-operative hydrophilic/hydrophobic hydrogen bonding, the varied strength of which emphasizes coherently the fact that PVP interacts both intra/intermolecularly with the respective surrounding fluid environment,30,31,34 enumerated by the sheer presence of selective NMR peaks (the complete range spectra is shown in S1, in the Supporting Information) observed in the range of 180−170 ppm (Figure 4), being related to the C resonance of carbonyl group of PVP.17,47,48 In polar aprotic DMF solvent, these peaks shift upfield by 2.2 ppm, indicative of partial double bond
Figure 3. High magnification (scale bar = 20 nm) TEM images clearly illustrate the increasing degree of anisotropy in the as-formed gold nanoparticles in their respective solvents as mentioned.
gies of these real gold nanostructures defy the conventional dimensionality and plasmonic hybridization criteria, which in turn impose a challenging task for modeling/simulating their subtle optical features. Interestingly, such a wide range of gold nanostructure morphologies, depicting clearly the deviation from sphericity in the following order: methanol < chloroform < ethanol < propanol < DMF, as shown in the high magnification images (Figure 3), strongly implies the complexity of the polymer− solvent−metal ion interaction even under normal conditions. Besides the already known reducing ability of PVP in aqueous medium,27,28 its stimulating solvent adaptable feature in 23723
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Table 1. Chemical Shifts (in ppm) Observed for the Different Carbon Environments in PVPa ppm
ppm Δδ
system
δ C6
Δ δ1
PVP DMF-PVP CHCl3−PVP propanol− PVP ethanol−PVP DMF−PVP− Au CHCl3− PVP−Au propanol− PVP−Au ethanol− PVP−Au
175.410 173.190 175.520 175.838
2.220 0.110 0.428
175.820 173.061
0.410 2.349
175.560
Δδ Δ δ2
δ C1,C3
Δ δ1
Δ δ2
42.084 40.425 42.084 42.190
1.659 0.000 0.106
0.129
42.240 40.301
0.156 1.783
0.124
0.150
0.040
42.090
0.006
0.006
175.857
0.447
0.019
42.210
0.126
0.020
175.780
0.370
0.040
42.110
0.026
0.130
a Δδ1 and Δδ2 are the changes in chemical shift with respect to PVP and their respective PVP−solvent complex.
For PVP in chloroform, major vibrational peak position changes corresponding to CH2/CH, CO, and C−N coupled bands (as illustrated in Table 2) clearly indicates that the PVP molecule interacts with CHCl3 solvent effectively through N−C stretch vibrational bands of N−CO and N−C−C in the pyrrolidone ring further weakening its conjugation, thereby strengthening the carbonyl group as shown in Scheme 1B. For PVP in alcohol medium, the highly dynamic correlated hydrophilic (hydrogen bonding between CO in the pyrrolidone ring and hydroxyl group of alcohols) vs hydrophobic (bonding between methyl/methylene groups and polymer monomer chain)50 interaction (shown in Scheme 1C) essentially nullifies the overall observed changes in the PVP carbonyl peak. Nevertheless, as the PVP concentration fixed at 10 mM in all our synthesis procedures, the hydrophobic interaction seems to dominate, as indicated by the visible changes in the CH2/CH as well as C−N peaks (Table 2). Hence, PVP in respective solvents shows distinctly different behavior from that of pure polymer, with the hydrogen bonded solvents acting as cross-linking points/bridges among the entangled inter/intra molecular PVP chains, leading to the formation of a complex solvophoboic interacting network,31,32,34 the co-operative strength of which varies in the following order: DMF > propanol >ethanol > chloroform. When HAuCl4 is added to this PVP−solvent mixture with a high monomer to metal ratio (∼3250), donation of lone pair of electrons both from the intra-/interchain of the entangled PVP network to the as-formed dynamically unstable PVP−solvent− AuCl4− complex takes place; thus, sequential ligand substitution occurs in the square planar metal centered complex, disproportionating itself to yield Au0 species.37 These in situ formed small gold seed particles then compete with PVP over the respective fluid surroundings strongly inducing desolvation effects, thus allowing the stabilizer polymer be chemisorbed on to the seed surface in an intricate manner right from its immediate nucleation, thereby preventing random agglomeration by steric stabilization.37 These two synergetic effects, desolvation (highly dependent on solvent character) and polymer coordination interaction with the metal ion, predominate the kinetic reaction mechanism under normal conditions, resulting in sequential equilibration of the PVP−
Figure 4. Extended 13C NMR spectra depicting clearly the carbonyl C6 (a and c), C1, and C3 (b and d) carbon chemical shifts in different samples. The strong upfield chemical shift in DMF is attributed to its strong interaction with PVP, further leading to exotic 3-D flower/star like Au morphology as discussed in the text.
characteristics due to resonance effect, asserting the fact that along with PVP−DMF hydrogen bonding, dipole−dipole interaction is also predominant.49 In case of protic solvents like alcohols (propanol and ethanol) and chloroform, they shift downfield with little chemical shifts of 0.428, 0.410, and 0.110 ppm, respectively (Figure 4 and Table 1), clearly representing the dominant hydrogen bonded configuration, the further contrasting evidence of which arises from the minimal chemical shift in the case of C−H bond (at 42.07 ppm, see Figure 4 and Table 1) for propanol/ethanol/chloroform and a strong upfield chemical shift for DMF (similar to the case of dimethyl sulfoxide (DMSO) as shown in S2) signify the predominantly strong dipolar/hydrophilic interaction in the polar aprotic solvents. The selective FTIR peaks of PVP dispersed in different solvents in comparison with the pure PVP powder are presented in Figure 5 (full range spectra is shown in S3) and the individual peaks are listed in Table 2, illustrating the unusual complexing ability of PVP as shown in Scheme 1. For PVP in DMF, a gradual increase in C−N stretching related frequencies and a strong decrease in CO absorption frequency along with changes in other peaks (as shown in Table 2) are attributed to the dipole−dipole interaction as well as hydrogen bonding between DMF and PVP (i.e., weakening of carbonyl group of PVP molecule making the C−N bond stronger) as represented in Scheme 1A. 23724
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Figure 5. Vibrational peaks in the selected wavenumber ranges for (a) PVP, (b) DMF−PVP, (c) chlorofom−PVP, (d) ethanol−PVP, (e) 2propanol−PVP, (f) DMF−PVP−Au, (g) chlorofom−PVP−Au, (h) ethanol−PVP−Au, and (i) 2-propanol−PVP−Au are as shown. Substantial contributions from the carbonyl CO and nitrogen coupled with other chemical bonds can be clearly seen.
Table 2. Observed Vibrational Peaks for PVP (Under Present Experimental Conditions)a IR PVP
a
peak assignment
PVP− CHCl3
PVP−CHCl3− Au
PVP− EtOH
PVP− EtOH− Au
PVP− Prop
PVP− Prop−Au
PVP−DMF
2985,2955 2924 2874 1663
asym ring (CH2) stretch sym (CH2 ) chain C−H stretch sym (CO) stretch
2990 2925 2884 1672
2990 2925 2884 1672
2971 2922 2877 1666
2971 2922 2877 1666
2965 2928 2875 1665
2965 2928 2875 1665
2948 2928 2859 1642, 1667
1495 1463 1439 1423 1374,1319
ring (C−N) stretch ring (CH2) scissor + ring (C−N) stretch + (CH) bending
1493 1461 1437 1423 1373, 1317
1493 1461 1437 1423 1373, 1317
1495 1463 1443 1426 1379,1316
1495 1461 1441 1424 1375
1495 1461 1441 1424 1375
1495 1459 1429 1406 1383, 1316
1289 1215 1170
1289 1215 1170
1495 1463 1443 1426 1379, 1316 1294 1226 1173
1294 1226 1173
1289
1289
1162, 1130 1103, 1016
1162, 1130 1103, 1016
1285 1256 1166, 1150
951 895, 846, 816 785, 762, 746 668
952 895, 846, 816 785, 762, 748 669
1292 1229 1170
CH bend+ ring (CH2) wag+ (NC) stretch ring (CH2)wag+ (CN) stretch CH2 twist + NC stretch weak ring CH2 twist
1071
(CN) stretch
1049
1049
1087, 1047
1084,1041
934 846
(C−C) ring breathing C−C ring
930 893, 881, 846
736
(C−C) chain
933 880, 847, 806 763, 747
933 883, 846, 806 763, 747
652
(N−CO) bend
930 893, 881, 846 767, 751, 736, 726 657
769, 752, 744, 736, 723 657
PVP− DMF− Au
783, 733, 700
2948 2926 2861 1669, 1655 1497 1459 1430 1404 1383, 1316 1286 1255 1170, 1151 1088, 1063, 1017 933 894, 864, 845 733, 700
657
657
1091(s),1062,1018
932 893, 864, 843
All wave numbers are given in cm−1. The illustrated peaks in Figure 5 are bold marked for easy reference purpose.
molecular studies, as meticulously pointed in the case of formation of gold nanostars,37 aesthetically interpreting the importance of solvent−solute interactions in determining the final morphology of the metal nanoparticles. Concomitantly, the different coordination bonds formed by PVP in their respective solvent medium along with the redox potential of metal precursor ions initiate the reduction process themselves under normal conditions in the absence of external reducing agent or energy sources like temperature, microwave/ultrasonication, seed mediation etc., thereby constantly rejuvenating its chemical interaction with Au, proceeding from ionic to
solvent and PVP−solvent−Au spectroscopic data (Figures 4, 5, S1, and S3), effectively illustrating the cohesiveness of the wellknown ligand exchange mechanism of metal square planar complexes, even in the case of nonaqueous solvents. Thus, our combined NMR and FTIR results unmistakably exemplifies the significance of highly dynamic sensitivity of the PVP−solvent interaction, further elucidating the fact that pyrrolidone monomer units radically adheres to the Au metal surface, both in ionic as well as particulate nature, which has been largely misunderstood for the formation of various unstable oxidation byproducts,27,51 for lack of methodical kinetic 23725
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significant interaction between the PVP molecule and the Au particle core. Likewise, the N (1s) core level peak of pure PVP (399.6 eV)53 shifts asymmetrically to slightly higher binding energies (400.07, 400.16, and 399.89 eV) for the Au nanostructures in DMF, ethanol, and propanol respectively (Figure 6 and Table 3), directly depicting the selective participation of nitrogen in assimilating nucleation and growth of anisotropic gold nanostructures as evidenced from the TEM images (Figures 2 and 3). In terms of quantifying specific charge-transfer interaction of the polymeric stabilizer with the Au nanoparticles with high clarity, the observed C (1s) core level spectra were deconvoluted into three peaks by standard nonlinear curve fitting procedures, representing the C−C, C−N, and N−CO bonds of the PVP molecule (see Figure 6 and Table 3), respectively, the varied strength of their peak intensities directly affiliates the surrounding chemical environment around the polymer. These relative changes in individual C 1s components delineate the following interesting facts. For the Au nanostructure in DMF solvent, practically no BE shift occurs for C−N and N−CO peaks, but the C−C peak shifts to lower binding energy in comparison with pure PVP, clearly distinguishing the subtle changes observed in their respective N 1s and O 1s core level spectra, preferably asserting the fact that the combined dipolar/hydrophilic interaction of PVP molecules in the polar aprotic reaction medium primarily exercise a balanced electron density distribution around both nitrogen and oxygen components in the pyrrolidone ring but sensitizes the C−C component due to the implicit change in overall chemical environment. Similarly in ethanol solvent, the major contribution from the N−CO component in comparison with pure PVP indicates partial bond weakening upon chemisorption with Au, implying stronger interaction between the polymer-gold interface, which practically controls the aggregation/arrangement of reduced Au atoms, essentially dictating the defect induced formation of decahedral gold nanostructures. In case of propanol, minimal red shift in C−N and N−CO peaks along with a slight blue-shift in C−C peak (with increasing C−C percentage contribution as we go from DMF to ethanol to propanol) with respect to pure PVP signifies the fact that hydrophobic interaction dominates (Scheme 1C) which is in narrow coincidence with our FTIR results (Figure 5 and Table 2), intuitively suggesting the indirect but significant contribution from the monomeric chain of PVP in the design and controlled formation of complex multi/hyper-branched gold nanostructures.
Scheme 1. PVP−Solvent Interaction As in (A) DMF, (B) Chloroform, and (C) Alcohols Like Ethanol and Propanol
particulate form, simultaneously entrusting the bridging ligands mechanism (Scheme 2) to indeed evolve on its own, as vividly supported by the classical coordination chemistry model for metallic nanocrystals.34,37,49,52 Further, the careful surface chemical composition analysis of the as-prepared anisotropic gold nanostructures using XPS details the chemisorption of PVP ligand with different size/ shaped Au nanocrystals and their corresponding effective interaction, thereby providing novel insights into the quantitative role played by the individual components of PVP in seedless surface mediation under normal experimental conditions. Figure 6 shows a typical C1s, O 1s, N 1s, and Au 4f core level spectrum of the purified Au nanoparticle samples, which strongly corroborate the presence of PVP molecules on the metal surface, further in tune with the diverse interaction between PVP and various gold crystalline facets as being reported.53,54 Previous XPS studies on PVP-capped metal nanostructures have empirically identified that either oxygen or nitrogen atoms of the pyrrolidone unit in PVP molecule might interact with the metal surfaces, rendering stabilization of thermodynamically unfavorable size/shapes.53,55,56 It is generally believed that PVP interacts with metal nanoparticles through its O atom in the pyrrolidone ring by ligand-to-metal charge transfer interaction; while the direct participation of N atoms is less favored due to steric hindrance effects.54,57 In the O 1s region, the peak arising from the carbonyl (C O) oxygen for pure PVP53 (531.3 eV) cohesively red shifts to 532.35, 532.17, and 532.06 eV for the Au samples synthesized in DMF, ethanol, and propanol, respectively (Figure 6 and table 3), illustrating the fact that the electron density consistently decreases around the carbonyl (CO) oxygen due to
Scheme 2. Schematic Showing the Three Possible Modes of Interaction of PVP with the Metal Nanoparticle via (a) Oxygen, (b) Nitrogen, and (c) Both Nitrogen and Oxygen
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Figure 6. XPS spectra of PVP-capped Au nanoparticles (synthesized in different solvents as shown) illustrating the corresponding C 1s, N 1s, O 1s, and Au 4f regions.
Table 3. XPS Binding Energy Peak Positions Obtained from the Different Core Level Spectra
a
peak
DMF
ethanol
2-propanol
PVP
Au4f7/2 Au4f5/2 C1s (C−C) C1s (C−N) C1s (N−CO) N1s O1s
84.11 87.76 284.82 (45)a 285.80 287.81 400.07 532.48
84.10 87.77 285.01 (52)a 286.00 288.31 400.16 532.22
84.16 87.83 285.01 (63)a 286.27 287.93 399.89 532.06
285.12 286.00 287.80 399.7 531.35
Percentage contribution as extracted from the area of the C−C peak.
lighting the urgent need to critically understand the physicochemical aspects of PVP as natural reductant/stabilizer system in the absence of external energy resources, so as to systematically tune/design the complex structure−property relationship between the polymer PVP and the metal nanoparticles, thereby enhancing their plasmonic and photonic application potentials, as never seen before.
Hence, the present comprehensive XPS analysis shows that the electron cloud density unequivocally hovers around the N− CO region of the pyrrolidone ring of PVP through the socalled conjugation effect, confirming the impartial charge transfer from both the oxygen and nitrogen atom in the pyrrolidone ring when dispersed in DMF solvent, the dynamic equilibrium of which sequentially gets drifted through solvent specific intra/intermolecular redistribution aptly supported by the monomer chain component, particularly when PVP is dispersed in alcohol environments. Moreover, in comparison with bulk Au atoms (84.0 eV for Au 4f 7/2 and 87.7 eV for Au 4f 5/2),53,54 the BE of Au 4f7/2 core level peak arises at 84.11, 84.1, and 84.16 eV and that of Au 4f5/2 core level peak arises at 87.76, 87.77, and 87.83 eV, respectively, in DMF, ethanol, and propanol solvent medium (see Figure 6 and Table 3), the concurrent changes of which drastically differs from the XPS data on other molecular/ surfactant/polymer capped Au clusters/nanoparticles,53,54 retrospectively implying a complex charge transfer process enabled by the convoluted electron-donating ability of PVP to metal ions, leading to ample nucleation followed by selective aggregation/interfacial alignment of reduced Au0 species, duly reaffirming the quantitative role of PVP in intuitively evaluating the bridging ligands mechanism (Scheme 2) with reference to the surrounding dielectric environment, thereby initiating a whole new perspective in the synthesis as well as characterization of size/shape controlled metal nanostructures. In summary, our present work quantitatively illustrates that the cooperative PVP−solvent−metal ion interaction sequentially leads to the formation of highly stable and reproducible anisotropic size/shaped metal nanostructures, essentially high-
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ASSOCIATED CONTENT
S Supporting Information *
NMR and FTIR spectra are illustrated in Figures S1−S3. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS One of the authors, A.K., thanks CSIR for the NET-JRF fellowship. We sincerely thank the R&D Scheme, MTech (Nano) as well as the USIC of the University of Delhi, for materials characterization. We are grateful to Mr. Rahul Bhardwaj for TEM imaging, Mr. Deeraj for NMR data, and Dr. P. R. Selvakannan, postdoctoral fellow, RMIT, Australia, for help in XPS measurements. 23727
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dx.doi.org/10.1021/jp306952d | J. Phys. Chem. C 2012, 116, 23721−23728
